The pitot tube is well known as a means to measure pressure of a fluid flowing through a duct. In the performance test of a ventilator, for example, it has been usual that the cross-section of the duct is divided into a plurality of equal areas and the arithmetic mean of the measurement values obtained by the pitot tube at the respective areas is calculated to obtain the average pressure (transverse cross-section division into equal areas utilizing pitot tube, JIS B 8330).
Certainly such procedure is appropriate when high accuracy is demanded such as in the performance test of a ventilator, but various problems are often encountered in the actual field of operation, for example, in an air conditioning equipment room. Specifically, this procedure requires a linear duct portion as a sum of such portions in front of and behind the measurement position, which is approximately ten times the duct diameter, to stabilize the fluid steam. However, such a space is often unavailable in a field of operation such as an air conditioning equipment room. Furthermore, it is difficult to establish the proper position as well as the proper angle of the pitot tube relative to the flow stream and errors often occur due to personal factors. A true mean value cannot be obtained unless the measurements at the respective areas are made simultaneously since the pressure distribution over the whole cross-section varies.
To overcome these problems, an apparatus has already been proposed, in which stationary tubular stream stabilizers provided with openings in the flow direction are located in association with the respective measurement points defined by the above procedure of transverse cross-section division into equal areas utilizing a pitot tube. Total pressure sensors and static pressure sensors are arranged on the downstream ends of the respective tubular stream stabilizers. The total pressure sensors associated with the respective tubular stream stabilizers communicate with one another by a total pressure manifold while the static pressure sensors associated with the respective tubular stream stabilizers similarly communicate with one another by a static pressure manifold. These manifolds include small tubes adapted to average total pressure and static pressure, respectively, and the total pressure is measured from said total pressure manifold while the static pressure is measured from the static pressure manifold.
With such apparatus of the prior art, however, the respective manifolds must be constructed in the form of complicated double pipes and the tubular stream stabilizers must be sufficiently long to minimize possible errors in measurement. This results in a total length of the apparatus which is inconveniently long.
With this apparatus of the prior art, the flow rate of the fluid is adjusted manually or automatically by opening and closing the damper blades on the basis of the values measured by the instruments. However, the instrument which measures the flow rate and the damper blades which adjust the flow rate are separately installed so that the distance from the instrument to the damper is disadvantageously large and, in consequence, an unacceptably long time delay occurs from the adjustment of the damper to the moment at which a change in the flow rate actually appears on the instrument. Thus, the flow rate adjustment has been troublesome with such apparatus of the prior art.